Imaging system employing ions

ABSTRACT

In the formation of visible copies of an image, the use of an ion modulating array provided with an asymmetrical photosensitive coating together with means to electrostatically develop the ion image.

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Finland et .211,

[ Sept, 25, 11973 1 IIMIAGING SYSTEM IEMIPLOYING HQNS [75] Inventors:Richard A. Fotland, Warrensville;

Virgil E. Strauglnan, Euclid, both of Ohio [73] Assignee: Horizonsllncorporated, a Division of Horizons Research Incorporated, Cleveland,Ohio [22] Filed: Aug. 2, 11972 [2]] Appl. No.: 275,674

Related US. Application Data [63] Continuation-in-part of Ser. No.178,521, Aug. 27,

[52] 11.5. CI 355/3, 96/1 R, 117/17.5, 355/16 [51] llnt. Cl (503g 15/00[58] Field of Search 355/3, l6, 17; 117/175; 96/1 R [56] ReferencesCited UNITED STATES PATENTS 3.582.206 6/1971 Burdige 355/16 Frank 355/3Cleare 355/17 X FOREIGN PATENTS OR APPLICATIONS 1,156,308 10/1963Germany 96/1 R OTHER PUBLICATIONS Defensive Publication, T879,0l0, L. F.Frank, Oct. I970, Exposure Latitude in Electrophotographic Systerns.

Primary Examiner-Robert P. Greiner Att0rneyLawrence 1. Field [57]ABSTRACT In the formation of visible copies of an image, the use of anion modulating array provided with an asymmetrical photosensitivecoating together with means to electrostatically develop the ion image.

35 Claims, 12 Drawing Figures PATENTEU 2 1 3 FIG. I.

FIG. 4A.

IMAGING SYSTEM IEMILOYING IONS This application is a continuation inpart of our earlier filed application Ser. No. 178,521 filed Aug. 27,1971.

This invention relates to image reproduction and more particularly to amethod and apparatus for the efficient formation of ion patternscorresponding to an optical image.

In conventional plain paper electrostatic photography, an insulatingphotoconductor is charged with a corona source of ions, exposed, thecharge image developed with toner, the developed image transferred toplain paper, and finally, the toned image is fixed, generally by fusing.After the transfer operation, the residual image is erased from thesurface of the photoconductor and the photoconductor is cleaned inpreparation of a repetition of the process. Although employing plainpaper, this process is complicated by the requirement for a number ofdifferent machine operations. In addition, the photoconductor sufferswear over a period of time, since the surface of the photoconductor isrepeatedly rubbed by toner particles, cleaning brushes and papersurfaces.

A related process employs a photoconductively coated conducting paper.The photoconductor, generally zinc oxide (although organicphotoconductors may be employed), is first charged, then exposed, andthe image toned. Here the photo-conductor is not reusable and thus thewear and tear restrictions in the aforementioned process are eliminated.In addition, the machine operation, requiring four steps, is simplified.The disadvantage of this process is associated with the requirement forcoating the paper with a photoconductor. These photoconductively coatedpapers are significantly more expensive than plain uncoated paper. Inaddition, because of the heavy photoconductor coating (the coatingweight generally amounting to pounds per 3,000 ft ream), the papers areheavy and have a feel quite different from plain paper.

A principal object of the present invention is to simplify theconventional plain paper electrophotographic process and the apparatusby which it is carried out.

Another object of the invention is to provide an image reproductionmethod wherein there is no physical contact of the photoconductor witheither developer or paper.

In addition to having the advantages of eliminating photoconductor wearand simplifying the number of machine operations, the method andapparatus of this invention do not require a photoconductively coatedpaper. In comparison therefore to electrostatic copy processes employingphotoconductive paper, the process of this invention has the advantageof lower paper cost through the use of a dielectric coated paper whichhas the feel, weight and appearance of a plain bond pa per.

Another object of the invention is to provide an image copying meanswhereby photoconductor defects, attracted dust and the like do notappear in the final copy; these defects being integrated out during anexposure.

In the present invention, a fine mesh screen or grid coated with aphotoconductor is employed to spatially modulate a flow of ions inaccordance with an optical image projected onto said fine mesh screen orgrid.

U.S. Pat. No. 3,220,324 discloses an apparatus and method of forming anelectrostatic charge pattern conforming to an optical image on achargeable member which includes an electrically conductive screenhaving a photoconductive layer thereon. In following the teachings ofthis patent, it is found that the control ratio, i.e., the ratio of ioncurrent passing through the screen between nonilluminated andilluminated conditions, is quite low. Under optimum conditions, thisratio is near 2.

According to the present invention, a conductive screen or aperturedplate which is coated asymmetrically with a photoconductor is utilized.Asymmetry, as employed in this sense, refers to a variation inphotoconductor thickness with position around the periphery of theapertures in the screen or plate. (Presence and absence ofphotoconductor is an extreme case of thickness variation.) As aconsequence of this asymmetry, it is possible to obtain a contrast ratioof several hundred.

One preferred embodiment of the present invention employs an insulatingscreen woven from a Nylon or Dacron monofilament which is first coatedwith an electrically conductive material and then coated, in anasymmetrical fashion, with a photoconductor.

The screen structure, the method of forming the screen and the apparatusfor utilizing the screen for the formation of visible images will bemore fully apparent from the description which follows taken with thedrawings in which:

FIG. 1 is a schematic view of an apparatus for preparing electrostaticcharge images, corresponding to a projected optical image, upon an imagereceptive surface;

FIGS. 2, 2A and 2B are cross section views ofa modulating conductivescreen illustrating the asymmetric nature of the photoconductive coatingthereon;

FIG. 3 is a similar cross section illustrating the geometry of adielectric or insulating screen coated asymmetrically with both aconducting layer and a photoconductor;

FIG. 4 is a fragmentary view of a section through a metal plate having aplurality of apertures and which is asymmetrically coated with aphotoconductor;

FIGS. 3A and 4A are enlarged views, in section, of portions of FIGS. 3and 4;

FIG. 5 illustrates schematically a means for moving thephotoconductively coated screen and the corona wires during an exposure;

FIG. 6 illustrates a means for continuously supplying a fresh modulatingscreen during the operation of a copy device;

FIG. 7 illustrates a modification including a photoconductively coatedscreen in copy operations wherein the final image is formed upon plainpaper, that is, paper which is not capable of sustaining a charge image;and

FIG. 8 illustrates an embodiment of the invention which employs a screengrid to electrically isolate a chargeable members surface potential fromthe screen potential.

Referring now to FIG. 1, illustrating an apparatus for preparingelectrostatic images on an image receptor surface, the apparatuscomprises an electrically conductive platen 10 upon which is supported aconducting paper 12 having a thin dielectric coating 114. A coronamodulating screen, grid or aperture plate 16 controls the ion currentreaching the surface of the dielectric paper in accordance with anoptical image projected onto element 16. A corona source is provided,which may comprise one or more fine wires I8. The corona operatingpotential is supplied by power supply 20. The paper support substrate 10is maintained at a selected potential provided by power supply 21.Electronic controls 24 provide a means for simultaneously turning onpower supplies 20, 21 and an illumination source for a projector 22.Projector 22 provides the image which is to be copied; this image beingfocused upon screen 16.

Although in this embodiment the optical image is provided by a projectorsuch as might be employed in the projection of microfilm images toobtain hard copy, it will be understood that projector 22 could bereplaced by a cathode-ray tube display using a projection lens system orby an original document support plus a projection system forconventional office copy, or any other suitable source of optical imagedepending upon the application of the apparatus. It should also beunderstood that, although these examples herein employ an optical(light) exposure, the input to the screen may consist of alternate formsof energy to which the photoconductor employed exhibits sensitivity.These other radiations generally include X-rays, gamma rays, and alphaand beta particles.

A single corona wire 18 is shown in FIG. 1. In order to provide auniform corona over a large area, a plurality of corona wires may beutilized all connected in parallel to power supply 20. In order toprovide sufficient corona current, the corona wire diameter should beless than 10 mils and to simplify handling of the wire, the wirediameter should be greater than 1 mil. A preferable wire diameter forthis application is 2 mils. Using a single corona wire spacedapproximately 1 inch above modulating screen 16, uniform charging, inaccordance with the projected optical image, of the dielectric paperoccurs over an area equal to the length of the corona wire and adistance between 1 and 2 inches normal to the direction of the coronawire at the paper. In order to provide for more uniform charging, thecorona wire(s) may be moved, in a plane parallel to the screen, duringthe exposure.

A dielectric paper 12 is shown in FIG. 1, such papers being availablefrom a variety of paper mills and being employed widely in high speedcomputer printers and recorders. The dielectric coated paper may bereplaced with any of a wide variety of plastic films ranging inthickness of 0.1 to mils. Images have been successfully formed on bothpolyester and acetate films; and, indeed, any film which has adielectric relaxation time in excess of a few seconds and which fallswithin the aforementioned thickness range may be employed in theapparatus of FIG. 1.

Since, under conditions of toner development to completion, the finalimage density is proportional to the charge density on the dielectriccoating; variations in dielectric coating thickness do not lead tovariations in the final image density and thus the uniformity of thedielectric coating is not critical in this application.

Means for mechanically transporting the dielectric paper or plastic filmunder the corona modulating screen, maintaining said paper (film)stationary during the exposure, and then removing the paper from theimaging station are not shown in FIG. 1; these mechanical features beingwell known to those skilled in the art. FIG. 1 shows the coronamodulating screen maintained at ground potential. In this event, thepotential on the corona wire 18 and backing plate must be opposite inpolarity. Thus, if the corona wire is maintained at a positivepotential, the backing plate must be maintained at a negative potentialso that positive ions emitted from the corona wire are accelerated tothe dielectric paper after passing through the meshes of screen 16.Alternately, the backing plate 10 might be main tained at groundpotential, screen 16 at a positive potential, and corona wire 18 at aneven higher positive potential.

A single corona wire may be mounted upon a carriage and, during anexposure, the wire may be made to traverse the screen thus providing auniform corona current exposure at the screen. In this case, a scanningimaging system, well known in the art and employed in a number ofcommercial copy machines, may be utilized to project a traveling sectionof the image at the corona wire.

The potential required between corona wire 18 and screen 16 must be atleast sufficient to initiate a corona current, i.e., at least 4 to 5 kv.The higher the potential the greater the ion current and hence the morerapidly dielectric paper may be charged and the lower the requiredexposure time. The upper limit of corona potential is realized whensparking occurs between corona wire 18 and screen 16. This is, ofcourse, a function of the spacing between 16 and 18. Corona potentialsas high as 25 kv have been successfully employed in this invention.

The potential required between screen 16 and backing plate 10 dependsupon the spacing between said members and the required resolution of theelectrostatic image formed on the charge supporting member. If thepotential for a given spacing is too high, sparking will occur betweenthe chargeable member and screen 16. Furthermore, at high potentials fora given spacing, the resolution of the charge image is sufficiently highso that a screen pattern corresponding to the screen 16 is observed inthe charge pattern laid down on the chargeable member. A preferredelectric field, in this region, is 20 kv per inch. This corresponds toan applied potential of 10 kv at a A inch spacing or 1 kv at a 50 milspacing. At this electric field, the corona cur rent passing throughscreen 16 and onto the chargeable member follows the field linesufficiently well so that a resolution of 6 to 10 line-pairs/mm isreadily obtained with screens having from 325 to 500 meshes per inch. Atelectric fields in the range of 50 to kv per inch, sparking occasionallyoccurs and the screen mesh pattern appears in the image. At fields belowapproximately 3 kv per inch, ion spreading is observed with subsequentdegradation of image resolution.

The exposure times required are a complicated function of the coronavoltage, corona-to-screen spacing, light intensity at the screen, natureof the photoconductor, and also the nature of the charge receivingmember and the type of development employed in converting theelectrostatic image into a visible image. In general, the requiredscreen illumination ranges from 1 to 50 ft.-candles of tungstenillumination and the exposure times range from 0.1 to 3 seconds.

FIG. 2 is a cross-sectional view of a wire mesh screen coated with aphotoconductor. The wire mesh 30 may be formed of any available metal oralloy, typical materials including brass, stainless steel, aluminum orphosphor bronze. The mesh size, i.e. the numbers of wires per linearinch, may range from 100 to 1,000. A 200 mesh screen will provide aresolution of 2 to 4 linepairs/mm while a 500 mesh screen is capable ofproviding 7 to 14 line-pairs/mm. The photoconductive coating 32 is shownhere as being offset by an angle of 45 from the normal. Hence one sideof the mesh openings will have a thicker photoconductive coating thanthe other side (said other side may have no such coating at all). Byforming the photoconductor in an asymmetrical manner such as this, muchhigher contrast ratios, i.e., ion current transmissivity ratio betweendark and light areas, are obtained in comparison to when thephotoconductor either totally surrounds the screen or is evaporatednormal to the plane of the screen. Increases in contrast ratio areobserved at deposition angles from the normal between 15 and 75, theregion of to 60 providing the highest contrast ratios.

One preferred way of applying the photoconductor to the screen 30 is byvacuum vapor deposition. The material to be vaporized is placed in acrucible or metal container which is electrically heated. The screen tobe coated is supported above the crucible at an angle, generally 45,with the normal. The orientation of the weave of this screen is notcritical. Thus, either the warp or Woof of the weave may be parallel tothe ground or run at any angle to the ground without adversely affectingthe contrast ratio.

In FIG. 2A, a single element of the array is shown, in section, showingthe manner in which the photosensitive coating 32 is disposed on thebase 30 in order to produce the asymmetry relative to the mesh apertureperipheries. FIG. 3A is a similar view showing the disposition of boththe photoconductive coating 37 and the electrically conductive coating36 on the insulating filament base 34. FIG. 4A is a similar view of thearray in FIG. 4.

FIG. 2B is a diagrammatic cross section 'of another suitableasymmetrical grid structure. Here, a large number of fine wires 30 arestretched to form a parallel wire grid. Typical wire diameters are 1 miland the center-to-center spacings 2.5 mils. Only alternate wires arecoated with a photoconductive layer 32. These alternate wires may beprecoated by dipping in a molten selenium bath, or by vacuum vapordeposition, or any of a number of techniques for providing suchcoatings. The grid is formed by simultaneously winding the two wires(the one photoconductively coated and the other bare) on a mandrel toform the precise required grid structure.

FIG. 3 shows another embodiment of the present invention; the screen 34Bbeing fabricated from an insulating material. Typical insulatingmaterials employed in this invention are woven fabrics consisting ofmonofilament polypropylene, polyester or polyamide. Such woven fabricscreens are available in mesh sizes to over 325 mesh, are extremelystrong and are much less expensive than corresponding metal wovenscreens. In FIG. 3A, the dielectric mesh 34 is shown having a conductivecoating 36 on the bottom and a photoconductive coating 37 on the top andoffset from the normal by 45, the two coatings overlapping or otherwisetouching so as to produce an electrical connection therebetween. Theconductive layer is preferably formed by vacuum vapor deposition of anelectrical conductor, such as aluminum, gold or Nichrome.

Satisfactory results are obtained if the woven screen is replaced by agrid of closely spaced wires; the wires being formed either fromconducting material, as illustrated in FIG. 2, or from insulatingmonofilaments which are subsequently coated with a conductive material,as shown in FIG. 3.

An alternate photoconductor support is shown in FIG. 4, which is across-sectional view of an aperture plate. The supporting plate 38,having a thickness in the range of l to 5 mils, may be fabricated byetching a plurality of holes through the surface and then coating thematerial with a photoconductor at an angle from the normal as shown inFIG. 4. Alternately, the plate 33 may be fabricated from a plastic sheetwhich also contains a plurality of holes etched in the surface. In thecase of an insulating support sheet, a conductor would be deposited onthe sides of the holes and bottom of the plate in a manner similar tothat shown in FIG. 3.

The mesh screens may be woven with either a plain square weave or atwill square weave. With a twill square weave, however, the resolutionin one direction is degraded slightly.

In many cases, a higher contrast ratio is observed if the screen ismounted so that the photoconductor coated side faces the corona wires. Asomewhat lower contrast is obtained with the photoconductor coated sidefacing away from the corona wires.

One of the many advantages of the present invention in comparison toconventional electrostatic photographic systems involves a relaxation inthe requirements for high dark resistivity of the photoconductor. Atypical selenium xerographic plate or drum has a capacity close to pf/cmIf such a plate is charged to 500 volts and the allowable voltage decaymust be 100 volts or less in a period of 1 second (the minimum timeinterval between charging and image development), then a simplecalculation will show that the dark current through the plate must beless than 10' amp/cm, or the plate dark resistance must be in excess of5 X 10" ohm/cm of plate area. In a typical screen modulation apparatus,as described herein, the corona current to the screen might be in therange of 3 X 10 amp/cm In order to provide effective modulation of thescreen corona current, it is estimated that a voltage drop of at least100 volts is required across the photoconductor coating of the screen.Thus, the photoconductor resistance in the dark must be in excess of 3 X10 ohm/cm of the screen area. This represents a 1,000 fold reduction inthe maximum dark resistance of the photoconductor coating the screen incomparison to photoconductors employed in conventional electrostaticphotography. The relaxation of this constraint permits the utilizationof a much wider range of photoconductor materials, particularly thosehaving higher sensitivity and/or extended red response. Evaporatedphotoconductors such as zinc cadmium sulfide, zinc cadmium selenide andcadmium sulfide, which, in the vapor deposited form, have too low aresistivity for conventional electrostatic photography are suitable forpreparing screens in the manner described in this invention. Inaddition, the selenium alloys having extended red light response such asselenium-tellurium alloys containing more than 10 percent tellurium andselenium-arsenic alloys containing at least 50 percent arsenic may alsobe employed in this invention.

Although the asymmetrical photoconductor deposition onto either aconducting or nonconducting screen may be readily carried out by vacuumvapor deposition, it is also possible to prepare photoconductive coatedscreens using photoconductor binder layers.

One preferred method of forming such a screen is to spray, using an airgun, the photoconductor binder layer material on the screen; the spraybeing directed onto the screen at an angle. Either suitably doped anddye sensitized zinc oxide or doped cadmium sulfide dispersed in asuitable solvent with any of a number of appropriate binders may besprayed onto the screen or mesh at the appropriate angle to form aneffective asymmetrical ion control screen. As previously indicated,because of the requirement for a relaxation of the dark resistivityrequirement, the concentration of photoconductor pigment in the binderthat may be employed on screens is significantly higher than that whichmust be utilized for the standard electrophotographic processes. Thispermits the fabrication of higher photosensitivity surfaces.

Another alternate approach for preparing binder layer screens involvessettling the powder onto a screen held at an angle away from thehorizontal; such settling operation being carried out in a liquid bath.The settling operation is carried out by first supporting a screen, atan angle of approximately 45 to the horizontal, at the bottom of a largebeaker. A suspension of photoconductor particles in a fluid is pouredinto the beaker and the particles are allowed to settle onto the screenunder the influence of gravity. The binder may be either dispersed oractually dissolved in the fluid. After the particles have all settledonto the screen, the fluid is siphoned out of the beaker and the screenallowed to dry in the beaker.

Organic photoconductors in a binder layer may also be employed asphotoconductors suitable for the present invention.

Photoconductors possessing a memory or high fatigue, such as certaintypes of ZnO, may be employed with this invention if it is desirable toseparate the exposure from the actual time of charging an ion receptormember.

As previously mentioned, the screen pattern on the chargeable member maybe eliminated by operating at electric fields sufficiently low so thatthe screen is not resolved on the chargeable member. In inexpensivecommercially available wire or plastic monofilament screens having verysmall mesh sizes or high mesh counts, the weave is found to be somewhatnonuniform, i.e., there are small random variations in mesh spacingwhich result in mesh irregularities appearing in the image developedupon the chargeable member. These small irregularities, which appear asmesh lines in the copy, may be eliminated by moving the screen over avery slight distance during the exposure. One manner of effecting thismotion is illustrated schematically in FIG. 5. Here, the modulatingscreen 16 and a series of corona wires 18 are mounted together on arigid framework 40. This framework is supported in such a manner that itmay be translated transversely from left to right. The frame is alsospring loaded so that it is urged against cam 42 which is driven by lowspeed motor 44. Motor M is energized during the exposure so that thecorona wires and screen move relative to the dielectric receptor sheetduring the exposure. Motions as small as 0.1 inch are generallysufficient to eliminate all screens nonuniformities from the developedimage. It has been found that the maximum screen velocity during anexposure is approximately 1 to 2 inches per second before image smearoccurs, depending upon the intensity of the corona current to the screenand the nature of the response time of the photoconductive coating. Thescreen may be advantageously moved in two directions as by a circular orfigure eight motion. In addition to eliminating screen variations fromthe developed image, the technique of moving the screen during anexposure also provides the advantage of eliminating the development ofother screen defects such as random dirt and dust which settle upon thescreen. The screen motion, in order to eliminate irregularities in thedirection of both screen wires or monofilaments, should be such that themotion is not in the direction of either wire or monofilament. Apreferred direction of the motion is at an angle of 45 with each wire.Besides a linear motion, orbital motion or a zigzag motion may beemployed to successfully eliminate screen nonuniformities from the imageand mechanism in place of cam 42 to provide such motions is readilyavailable.

FIG. 6 illustrates a means for moving the screen during an exposure andfor simultaneously providing for the replenishment of new screen withinthe exposure region. As shown screen 46, having a width of between 4 and18 inches, depending upon the copy size desired, and coatedasymmetrically with a photoconductor is supplied from supply drum 45. Atakeup drum 47 collects the screen after it passes through the exposurearea. In operation, during each exposure, the screen advances a distanceof approximately 1/16 inch. In this way for every few hundred copiesthat are made, the screen is completely replaced with previously unusedscreen from drum 45. With many vacuum coated screens, several thousandsof copies have been formed from each screen with no degradation of theprocess.

Thus far, in the description of this invention, the use of dielectriccoated paper or plastic films have been indicated. In addition to thesematerials, papers fabricated from plastic (the so-called plastic papers)may also be employed in this invention. Conventional plain papersfabricated from cellulose generally contain a sufficient quantity ofmoisture and free ions so that these papers will not support anelectrostatic charge image for the time intervals required to practicethe present invention. By suitably treating plain papers, however,images may be formed using this invention. Plain bond papers or plainblade-coated papers may be rendered sufficiently insulating by firstheating the papers to a temperature of between and 200 C for a period ofa few seconds. This may be carried out in a small oven. Immediatelyafter removal from the oven and while the paper is cooling down, thepaper is wet with a hydrocarbon. A preferred material for thisapplication is the aliphatic hydrocarbon solvent known under thetradename of Isopar, manufactured and marketed by the Humble Oil &Refining Company. Paper so treated is capable of sustaining anelectrostatic charge on the surface for long periods of time. A lateralsurface conductivity is still present, however, so that once a chargeimage has been placed on the surface of the paper, the development mustbe carried out within a period of 1 to 2 seconds if excessive resolutiondegradation is to be avoided.

The latent electrostatic image formed by the corona modulation screenmay also be employed in recording an image using a deformablethermoplastic film composed of polystyrene, Staybelite, Piccolastic orother deformable synthetic polymer material. After the formation of anelectrostatic image on the film surface,

the latent image is developed by softening the film by exposure toeither heat or solvent vapors as is well known in the art.

In addition to providing a permanent image, the corona modulatingscreens of this invention may also be employed with electric fieldsensitive cholesteric liquid crystal films in display applications.Here, the dielectric coated paper of FIG. l is replaced by a liquidcrystal film and the support platen ill replaced by a glass sheet havinga transparent conductive coating on the side adjacent the film. Underthe influence of an electric field provided by ions reaching the freesurface of the liquid crystal film, the optical scattering and/orreflective properties of said film are modified, leading to theformation of a visible display on the film. Cholesteric materialssuitable for this application are described in British Patents 1,123,117and 1,167,486, and also by L. Melamed and D. Rubin, Appli. Phys. Lett.16, 4, 149 (1970) and by J. J. Wysocki, J. Adams, and W. Haas, Phys.Rev. Lett. 20, 19, 1024 (1968).

FIG. '7 illustrates an apparatus in which the toned image is firstformed on an intermediate endless belt and subsequently transferred to aplain paper sheet or web. In this drawing an endless plastic belt 62,preferably fabricated of polyester and containing a conductive coatingon the inside surface is supported on rollers 6d and 71]). Anelectrostatic image is formed on this belt using a corona modulationscreen, corona wire and projection source in a manner similar to thatshown in FIG. 11. After the electrostatic image has been formed, it isdeveloped by immersion in liquid developer tank 66 in the region betweenroller 64 and '70, the developed image is partially dried and thenoffset onto a paper sheet or web 72 as the paper and plastic film areheld in contact by rollers 76]]. The image is fixed on the paper andsome residual solvent removed as the paper is heated by radiant heater'74. Cleaning brush re removes residual toner from the plastic endlessbelt.

Rather than employ the endless plastic belt (as shown in FIG. 7), aconductive drum coated with a hard insulating surface, such as aglass-based enamel, may be employed. in apparatus employing a drum, theoperational steps are the same. The electrostatic image is formed usinga corona wire and a corona modulating screen; the electrostatic image istoned, employing either a dry or liquid electrostatic developer; theimage transferred by offset to a plain paper sheet or web; and the drumcleaned. This apparatus is rather complicated but does possess severaladvantages over conventional plain paper electrostatic photography. Aprincipal advantage is the fact that the photoconductor is never inphysical contact with either a developer material or paper and, hence,is not subject to the usual wear which occurs in standardelectrophotographic plain paper copies. An insulating surface enameleddrum possesses a hard abrasion resistant surface and hence has a liftsignificantly greater than a typical selenium drum. lDevices employingan endless plastic belt would be subject to a higher degree of wear;however, the belt may be readily changed and is relatively inexpensivecompared to a selenium drum.

It has been found that when the surface of a charge receptor member ischarged to voltages high in comparison to the voltage existing betweenscreen in and backing plate id, image distortion occurs. This distortionarises from fields at the surface of the chargeable member; thesefields, existing between a charged and uncharged region. This results incorona generated ion beam bending in a manner so as to reduce the widthof uncharged lines. Secondly, when a high potential is built up in asignificantly large area, a reduction in the local field immediatelybelow screen 16 occurs in this region with subsequent diffusion of ionspassing through the screen in this region. This effect is not seriousfor low charging voltages, particularly when high potentials are appliedbetween screen 116 and backing electrode 110. In charging relativelythick plastic films, which requires high surface voltage potentials(several thousand volts, for example, in case of 3 to 5 mil polyester oracetate film) these distortions are observed.

A means for circumventing this problem is shown in FIG. 8. Thisapparatus is identical to that shown in FIG. 11 but includes a secondfind mesh screen 100 whose potential is established by power supply 102.This fine mesh conducting screen is spaced very close to the surface ofthe chargeable member, generally within a distance of 5 to 25 mils. Thescreen potential, as established by power supply 102, is maintainedbetween the potential of backing plate It) and screen 116. This screenserves the same function as a screen grid in a conventional tetrodeelectron vacuum tube; its function being to isolate the potential at thesurface of a chargeable member from potentials existing in the regionbetween screen 100 and screen 16. Thus, surface potentials may be builtup without resulting in the ion beam diffusion and distortions mentionedpreviously. We have found that, because of the high system resolution,moire patterns are formed in the image corresponding to screen meshoverlap between screen 100 and screen to. In order to eliminate thisproblem, screen Wt) may be vibrated or caused to move by employing amotor and cam assembly 1041 operating in a manner similar to that shownin FIG. 5.

The following examples illustrate the techniques of the method, processand apparatus described in this disclosure. These examples are not meantto be restrictive in any way, however.

EXAMPLE l A plain square weave W0 mesh phosphor bronze screen wasstretched over a square brass frame whose inside dimension was 4 incheson a side'and whose outside dimension was 5 inches. The phosphor bronzescreen was soft soldered onto the frame. The frame was mounted in avacuum coater an average distance of 12 inches from a quartz cruciblemounted in tantalum heater. The screen was inclined 45 from the normal.A charge of 30 grams of xerographic grade selenium was placed in theevaporation crucible. The system was evacuated to a pressure of 10' torrand the selenium evaporated from the boat onto the screen over a periodof 45 minutes. During the evaporation, the screen was heated, with anelectrical heater, to a temperature of C. The selenium coating thicknesswas found to be 25 microns.

The screen was removed from the vacuum evaporator and mounted in theapparatus shown in FIG. 1. A 6 inch corona wire comprised of a 3.5 milthick diameter platinum wire was supported a distance of 1 inch abovethe screen. The screen to conducting platen spacing was is inch.

The contrast ratio, defined here as the ratio between the ion current tothe conductive backing plate it with the photoconductive screen in thedark and ion current I H l with the same screen illuminated, wasdetermined by connecting a Keithley Model 600A electrometer betweenpaper supporting electrode llll and power supply 21. At acounterelectrode potential of 5 kv and a co- EV EQQEE 3 -25 350mg 955323@522: 2 25 2:35 2523 253a on 9.5 53:23 2683 2:53 on mesw coo 2 EQQE NcosEonEE 3 More 8536 353 323 M5205 2 c25 EEE w 2596a 2503 mw can 535:3583 2E0 NH we me w eoo 25 1 22725 2 :FEE m E e w fi: ed fim E wH mm s ES B M QV E :00 8 3 m m w d 2 2 6 3 3 6 39A 528 3 82m aouoneeee ona 502cc 55 36250 m no 26 25 aeoz 282 aeo 262 8 E8: da 1% 35.8: 8 3. de 2 6.222 5 2 50 :E a do 83: e2 d :5 m @52 m 8% 3 6 w 555 305 QE EQE .SQQS Moi:m wmBEEm nmw mm mm flEfiw :3 man mmfiEaum nmm ii u s nmm fifi u mammacaw mum couodw :3 man I flaig immww m mw nv nv mv llllllllllllllllllmv n I I l I I I I I I I I I I mw .Eflioq 5 fiwoa w N 3 4 E. bw IIIIII IUS$35 z I I S 5 z I E. I I I S. I I cw IIIIIIIIIIIIIIIIIII om IIIIIIIIIII I 0 v dag wu b n w 2 IIIIIIIIIIIIIIIIIII mm IIIIIIIIIIIIIIIIII 2IIIIIIIIIIIIIIIIII 2 IIIIIIIIIIIIIIIIII ow IIIIIIIIIIIIIIIIII omIIIIIIIIIIIIIIIIII 0m IIIIIIIIIIIIIIIIII om QG m Eu u v s sm 52 835 82633.5m new :4 553a: 856 eo fienat :53 eozfieneeo En=$ ozfieun EEW eo ioaeB265 desfionag E==ow 02:2: :o mono 815 E E HB Qw 322E 2 8 Q 63:28 EEE Eeu Ew E252 II... 3:85 akaeam 525: Binaam I 52515 EEe Qw d ozueoefiozm 3a m A. w u a m 2 m H HAQ E 0 5 0 5 0 5 2 2 3 4 6 6 rona potential of +16 kv, the dark current was 24 am- 5 peres and the current obtained whenthe screen was uniformly illuminated with tungsten illumination at alevel of 10 foot-candles was 0.3 pamperes. The contrast ratio was thus80. At a corona potential of +1 2 kv, the dark current was 1 1 pamperesand the light current 10 was 0.15 ,aamperes; yielding a contrast ratioof 75.

It may be seen from the aforementioned measurements that a highercontrast potential is obtained at lower corona potentials. In thisevent, however, the corona current is lower, and longer exposure timesare re- I5 quired to charge the dielectric paper. At a screen-paperseparation of 1% inch, an applied potential of 3 kv is sufficient toaccelerate the ions to the surface of a dielectric coated paper andstill maintain a resolution of 3 line-pairs/mm in the developed image.

Copies of a projected image were obtained by placing sheets ofdielectric coated paper on the counterelectrode 10. An image having ahigh-light brightness of 10 foot-candles was projected on the screenwith a simultaneous application of corona and counterelectrodepotentials; the total exposure time being 3 seconds. The paper was thenremoved from the counter-electrode and immersed in a beaker of liquidelectrostatic toner containing positively charged particles and having asolids concentration of 1 percent. Since the paper surface was chargedpositively and since the liquid developer toner particles are alsopositively charged, a reversal image was obtained. When the same imagewas projected on the screen and the resulting latent electro- 35 staticimage developed in liquid toner with negatively charged particles, thenthe developed image was a positive image, i.e. black characters on awhite background as exhibited by the positive original. After the paperwas removed from the developer, excess liquid was squeegeed from thesurface and the paper dried in an air stream which, in accordance with apreferred mode, was heated. The image was of high quality, havingnegligible background and a maximum density of 1.1. The development timewas 3 seconds.

EXAMPLE 2 (PRIOR ART) A selenium coated ion current modulating screenwas prepared in a manner identical to that of Example 1 with theexception that the evaporation was carried out with the screen mountednormal to the line of evaporation. When evaluated in the apparatus ofFIG. 1, it was found that the dark current was 7.3 ,uamperes and thecurrent, with an illumination level of 10 footcandles, was 4.6 namperes;providing a contrast ratio 5 5 of 1.6. A number of attempts were made toobtain satisfactory copies of the light image in a manner described inExample l. in no case was it possible to obtain a high contrast betweenlight and dark areas on the paper. High background levels were obtainedtogether with low image density.

Example 2 illustrates results obtained employing the teachings of U. S.Pat. No. 3,220,324. This example is included to indicate the advantagesrealized when one practices the teachings of the present invention.

The following examples illustrate the diversity of photoconductormaterials, deposition methods, and geometry of screen arrangementssuitable for utilization in the present invention.

.fqstsst sls EXAMPLE Illl (PRlOlR ART) in order to further demonstratethat symmetrically coated photoconductor screens are inferior toasymmetrically coated screens, a 6 inches X 6 inches 325 mesh stainlesssteel screen was selenium coated on both sides by a technique designedto produce a uniform or symmetrical deposit on the wires. The screen wasmounted in a motor driven rotating jig at a 45 angle with respect to theevaporating selenium direction. Average distance from boat to screen was15 inches, and 60 grams of selenium were evaporated from the boat heldat 260 C. Once the evaporations proceded to completion, the screen wasreversed at the same angular orientation and coated in an identicalmanner on the opposite side. This reversal of the same angularorientation resulted in uniform coverage of the screen wires withphotoconductor, producing a symmetrically coated screen. Test conditionswere: corona voltage, l4 kv at 1 inch spacing; screen at groundpotential; accelerating plate potential, 5 kv at k inch spacing; lightlevel 140 foot candles, steady state measurement. The contrast ratio wasfound to be only 2.1 on one side of the screen and 2.6 on the other.

EXAMPLE t2 7 Several screens were asymmetrically coated on both sideswith photoconductor, one example of which is de scribed below. A framed325 mesh stainless steel V screen with an area 50 cm was coated withselenium by vacuum deposition at an angle of 45, the boat charge being60 g. The screen was then reversed and rotated 180 and only 5 g ofselenium was evaporated onto the opposite side. This coating arrangementled to an exaggeration of the asymmetry of the deposit with respect tothe opening as viewed from either side of the screen. Steady state testmeasurements under the conditions noted in Example 11 showed thecontrast ratio of the 60 gram side of the screen to be 900216 ll fobtcandles. The opposite or thinner coated side indicated a lesser thoughrelatively high contrast ratio of 32 at 7 EXAMPLE 113 Several screenswere prepared to determine the effect of angle of deposition forachieving asymmetry. Type 304 plain weave, 325 mesh stainless steelscreen was stretched and cemented to aluminum frames 7 inches X 7 inches0.11). with 6 inches inches open screen area. iFive screens were cleanedby solvent vapor degreasing and subseqpently selenium coated b a u m evau der dsm qs -2 9as exceptfor arigle of deposition. Conditions were asfollows: substrate temperature, 90 C; selenium charge, 65 grams;target-to-boat distance, 18 in ches; evaporator boat temperature, 250 to270 C; pressure 2 ltorr. Deposition angles, as measured from the normal,were 30, 45, 60 and 75. The results are shown below in Table Ili. Whilethe angle is not criti% the optimum valueis between 30 and 15.Testcbnditions were: co

rona voltage, +14 kv at 1 inch spacing; screen at ground potential;accelerating plate potential, 5 kv at rt inch spacing; light level, 140foot candles while chopped at a rate of 100 cycles/minute.

TABLE II Angle of Deposition in 15 30 45 60 75 Contrast ratio Withoutintending to limit the present invention to a specific mechanism, it isbelieved that the asymmetrical ion current modulating screen functionsthrough the control of ion transport through the screen apertures by atransverse electric field, i.e., a field set up in a direction of theplane of the screen which arises due to the asymmetrical nature of thephotoconductivc coating on the screen.

While but a limited number of embodiments of the present invention havebeen here disclosed, it will be apparent that many variations may bemade therein without departing from the spirit of the invention asdefined in the following claims.

We claim:

1. In the formation of visible copies of an image, an apparatus formodulating the flow of an ion beam in conformance with an optical image,which apparatus includes:

an electrically conducting element having a series of ion-permablelocations distributed thereover in an array, and a layer ofphotosensitive material on said element and asymmetrically disposedabout said locations.

2. The apparatus of claim 1, wherein said array comprises a mesh screen,the openings through said screen comprising said ion-permeablelocations.

3. The apparatus of claim 2, wherein said mesh is formed of wire.

4. In the apparatus of claim 1, means operatively connected to saidelement for moving it substantially at right angles to the direction oftravel of said ion beam during operation of said apparatus.

5. In the combination of claim ll, a source of ions on one side of saidelement, a chargeable member on the other side thereof, and means forforming an optical image on said element.

6. The apparatus of claim 5, wherein said array comprises a mesh screen,the openings through said screen comprising said ion-permeablelocations.

7. The apparatus of claim 6, wherein said mesh is formed of wire.

8. In the apparatus of claim 5, means operatively connected to saidelement for moving it substantially at right angles to the directions oftravel of said ion beam during operation of said apparatus.

9. The combination of claim 5, in which said optical image is formed onthe side of said element facing said source of ions.

10. .The apparatus of claim 9, wherein said array comprises a meshscreen, the openings through said screen comprising said ion-permeablelocations.

111. The apparatus of claim 10, wherein said mesh is formed of wire.

112. In the apparatus of claim 9, means operatively connected to saidelement for moving it substantially at right angles to the direction oftravel of said ion beam during operation of said apparatus.

113. The apparatus of claim ll including, in addition:

a source of ions and an electric-field sensitive cholesteric liquidcrystal film, which are positioned on opposite sides of said element.

lld. The apparatus of claim ll including, in addition:

a source of ions and a deformable thermoplastic film which arepositioned on opposite sides of said element.

115. The apparatus of claim ll, in which said element is a non-conductorand is provided with a conductive layer thereon in electricallyconductive relation to said phstsu s ti s l y H6. The apparatus of claim15, in which said element comprises an organic filament woven meshscreen.

17. The apparatus of claim 15, in which said conductive layer is on oneside of said element and said photosensitive layer is on the other sidethereof, said layers making electrical connection with one another.

18. The apparatus of claim 1, wherein said element comprises a pluralityof strands arranged to define a plurality of openings between strands,said photosensitive layer being on said strands and having substantiallya crescent shape when viewed in a cross section taken perpendicular tothe axis of said strands.

19. The apparatus of claim 18, in which said photosensitive layer isdisposed of a sector of the periphery of said strands which iscircumferentially offset from the top-to-bottom direction of saidelement by between about 15 and 75.

20. The apparatus of claim 19, in which said angle is about 45.

21. A screen comprising a plurality of strands arranged to define aplurality of openings between strands, and a photosensitive layer onsaid strands arranged asymmetrically around said openings.

22. The screen of claim 21, wherein the photosensitive layer on thestrands, when viewed in a section taken as a plane perpendicular to theaxis of each strand, is a crescent shape.

23. The screen of claim 22, in which said photosensitive layer isdisposed on a sector of the periphery of said strands which iscircumferentially offset from the top-to-bottom direction of said screenby between about 15 and 75.

24. The screen of claim 23, in which said angle is about 45.

25. The screen of claim 21, wherein said screen is a woven screen andsaid strands are metallic.

26. The screen of claim 21, wherein said strands are nonmetallic andsaid strands are coated on a first portion of their outer surface with alayer of photosensitive material and on a second portion of their outersurface with an electrically conductive layer, said layers makingelectrical connection with one another.

27. The method of fabricating a screen to modulate its ions dischargedfrom a corona source and passing to an image receptor member in acopying apparatus, which comprises:

providing a flow of photosensitive material moving along a path having agiven direction;

positioning in said path a screen comprising an element having a seriesof recesses distributed over a face thereof, and

orienting said screen in said path so that the plane of said recessesmakes an angle with said given direction between about 15 and 28. Themethod of claim 27, in which said angle is about 45.

29. The apparatus of claim 1 wherein said element is an apertured plate.

30. The apparatus of claim 2 including means to supply fresh screenwithin an exposure region in order to provide for continued andrepetitive operation of said apparatus.

31. The apparatus of claim 1 including, in addition, an image receptorsurface, means to develop a visible image on said surface and means totransfer the visible image developed on said image receptor surface ontoa permanent record member.

32. The apparatus of claim 6 including, in addition, a second screenpositioned very closely adjacent to the chargeable member, and means tomaintain said second screen at a suitable potential.

33. The apparatus of claim 31 wherein said chargeable member is aconductive drum coated with an insulating layer.

34. The apparatus of claim 1 wherein said array is and endless belt.

35. The apparatus of claim 1 wherein the electrically conducting elementis comprised of a grid of parallel electrically conductive strands andalternate strands are coated with a photosensitive layer.

1. In the formation of visible copies of an image, an apparatus formoDulating the flow of an ion beam in conformance with an optical image,which apparatus includes: an electrically conducting element having aseries of ionpermable locations distributed thereover in an array, and alayer of photosensitive material on said element and asymmetricallydisposed about said locations.
 2. The apparatus of claim 1, wherein saidarray comprises a mesh screen, the openings through said screencomprising said ion-permeable locations.
 3. The apparatus of claim 2,wherein said mesh is formed of wire.
 4. In the apparatus of claim 1,means operatively connected to said element for moving it substantiallyat right angles to the direction of travel of said ion beam duringoperation of said apparatus.
 5. In the combination of claim 1, a sourceof ions on one side of said element, a chargeable member on the otherside thereof, and means for forming an optical image on said element. 6.The apparatus of claim 5, wherein said array comprises a mesh screen,the openings through said screen comprising said ion-permeablelocations.
 7. The apparatus of claim 6, wherein said mesh is formed ofwire.
 8. In the apparatus of claim 5, means operatively connected tosaid element for moving it substantially at right angles to thedirections of travel of said ion beam during operation of saidapparatus.
 9. The combination of claim 5, in which said optical image isformed on the side of said element facing said source of ions.
 10. Theapparatus of claim 9, wherein said array comprises a mesh screen, theopenings through said screen comprising said ion-permeable locations.11. The apparatus of claim 10, wherein said mesh is formed of wire. 12.In the apparatus of claim 9, means operatively connected to said elementfor moving it substantially at right angles to the direction of travelof said ion beam during operation of said apparatus.
 13. The apparatusof claim 1 including, in addition: a source of ions and anelectric-field sensitive cholesteric liquid crystal film, which arepositioned on opposite sides of said element.
 14. The apparatus of claim1 including, in addition: a source of ions and a deformablethermoplastic film which are positioned on opposite sides of saidelement.
 15. The apparatus of claim 1, in which said element is anon-conductor and is provided with a conductive layer thereon inelectrically conductive relation to said photosensitive layer.
 16. Theapparatus of claim 15, in which said element comprises an organicfilament woven mesh screen.
 17. The apparatus of claim 15, in which saidconductive layer is on one side of said element and said photosensitivelayer is on the other side thereof, said layers making electricalconnection with one another.
 18. The apparatus of claim 1, wherein saidelement comprises a plurality of strands arranged to define a pluralityof openings between strands, said photosensitive layer being on saidstrands and having substantially a crescent shape when viewed in a crosssection taken perpendicular to the axis of said strands.
 19. Theapparatus of claim 18, in which said photosensitive layer is disposed ofa sector of the periphery of said strands which is circumferentiallyoffset from the top-to-bottom direction of said element by between about15* and 75*.
 20. The apparatus of claim 19, in which said angle is about45*.
 21. A screen comprising a plurality of strands arranged to define aplurality of openings between strands, and a photosensitive layer onsaid strands arranged asymmetrically around said openings.
 22. Thescreen of claim 21, wherein the photosensitive layer on the strands,when viewed in a section taken as a plane perpendicular to the axis ofeach strand, is a crescent shape.
 23. The screen of claim 22, in whichsaid photosensitive layer is disposed on a sector of the periphery ofsaid strands which is circumferentially offset from the top-to-bottomdirection of said screen by between about 15* And 75*.
 24. The screen ofclaim 23, in which said angle is about 45*.
 25. The screen of claim 21,wherein said screen is a woven screen and said strands are metallic. 26.The screen of claim 21, wherein said strands are nonmetallic and saidstrands are coated on a first portion of their outer surface with alayer of photosensitive material and on a second portion of their outersurface with an electrically conductive layer, said layers makingelectrical connection with one another.
 27. The method of fabricating ascreen to modulate its ions discharged from a corona source and passingto an image receptor member in a copying apparatus, which comprises:providing a flow of photosensitive material moving along a path having agiven direction; positioning in said path a screen comprising an elementhaving a series of recesses distributed over a face thereof, andorienting said screen in said path so that the plane of said recessesmakes an angle with said given direction between about 15* and 75*. 28.The method of claim 27, in which said angle is about 45*.
 29. Theapparatus of claim 1 wherein said element is an apertured plate.
 30. Theapparatus of claim 2 including means to supply fresh screen within anexposure region in order to provide for continued and repetitiveoperation of said apparatus.
 31. The apparatus of claim 1 including, inaddition, an image receptor surface, means to develop a visible image onsaid surface and means to transfer the visible image developed on saidimage receptor surface onto a permanent record member.
 32. The apparatusof claim 6 including, in addition, a second screen positioned veryclosely adjacent to the chargeable member, and means to maintain saidsecond screen at a suitable potential.
 33. The apparatus of claim 31wherein said chargeable member is a conductive drum coated with aninsulating layer.
 34. The apparatus of claim 1 wherein said array is andendless belt.
 35. The apparatus of claim 1 wherein the electricallyconducting element is comprised of a grid of parallel electricallyconductive strands and alternate strands are coated with aphotosensitive layer.